Embodiments described herein relate to a hydrocarbon trap having improved absorption of cold-start engine emissions, and more particularly, to a hydrocarbon trap containing an acidic absorption material for improving absorption of low molecular weight hydrocarbons.
In recent years, considerable efforts have been made to reduce the level of hydrocarbon emissions from vehicle engines. For example, government regulations in the United States have restricted emissions of non-methane organic gas (NMOG) during cold-start of engines. Cold start engine emissions are recognized as a significant contributor to hydrocarbon exhaust emissions as most catalysts used in vehicle exhausts rely on the latent heat of the exhaust gas to become catalytically active. It has been estimated that 70 to 80% of the non-methane hydrocarbon emissions that escape conversion by the catalysts are emitted during the first two minutes after a cold start as a vehicle typically requires up to two minutes to supply sufficient heat for the catalyst to reach a light-off temperature, i.e., about 200° C. to 400° C. Ethylene and propylene comprisea large portion of NMOG in vehicle exhausts. Since these molecules have low boiling points and high vapor pressures, they tend to exit from the exhaust at the low temperatures experienced during cold start.
Hydrocarbon traps have been developed for reducing emissions during cold-start by trapping hydrocarbon (HC) emissions at low temperatures and releasing them at sufficiently elevated temperatures through a catalyzed overlayer or by passing the emissions to downstream catalysts for complete oxidation. Currently, zeolites or molecular sieves have been the most widely used absorption materials for hydrocarbon traps. However, low molecular weight molecules of hydrocarbons, such as ethylene, propylene, and ethanol, tend to be released from zeolites at temperatures below those required for oxidation by catalysts. In addition, the pore structure of zeolites tends to limit the size of hydrocarbon species that can be adsorbed.
Further, hydrocarbon traps utilizing zeolites tend to suffer from decreased thermal durability upon aging when used in an exhaust system. This is due, at least in part, to alumina in the zeolite being leached out of the zeolite in the presence of water vapor at high temperatures. This results in less absorptive capacity for low molecular weight hydrocarbons. For example, in traps which utilize a bottom zeolite layer and a top catalyst layer, the aging process presents several problems. The top catalyst requires a higher temperature for conversion of stored hydrocarbon emissions and, as the bottom zeolite layer becomes dealuminated, it releases stored hydrocarbon emissions at lower temperatures. Generally, as the SiO2/Al2O3 ratio of zeolite decreases, the thermal durability of the zeolite decreases. Likewise, zeolites with high SiO2/Al2O3 ratios typically have decreased absorptive capacity. In fact, by the time the vehicle reaches 150,000 miles, few hydrocarbons stored in the zeolite are retained until the required temperature is reached for catalytic conversion.
While metal ions such as Cu, Fe, or Ag may be added to the zeolite in hydrocarbon traps to enhance hydrocarbon absorption, such metal ions can have a negative effect on the storage and thermal durability of the trap.
Accordingly, there is a need in the art for a hydrocarbon trap which can improve the efficiency of reduction of cold-start engine emissions including NMOG and particularly, low molecular weight hydrocarbons. There is also a need for a hydrocarbon trap which is thermally durable and long-lived.